Television transmitting apparatus



March 7, 1939. J, D'MCGEE HAL 2,149,455

TELEVISION TRANSMITTING APPARATUS Filed Oct. 23, 1935 2 Sheets-Sheet 1 March 7, 1939. I A D MCGEE r 2,149,455

TELEVIS ION TRANSMITTING APPARATUS Filed Oct. 23', 1935 2 Sheets-Sheet 2 Patented Mar. 7,

James" Dryer George lleGee, sin-r mm- I. n amam'rus Illing, London, and

liondmln'lmlstofleckickblusl- Limitcllhyestling- InniaoompnnyodGreatllrltnin October 23, 1835, Serial No. 46,254 'IIIGMWWM,I934

The present invention relatm to transmitting apparatus.

In the Journal of the Institution of Electrical Engineers for October 1933, page 437, there is described a method of transmitting images of an object to a distance with the aid of a cathode ray tube. Such a .tube, which is there an "lconoscope, comprises a sealed envelope having a narrow neck portion flaring out into a bulbous portion. Within the portion is disposed a mosaic screen in the form of an insulating plate bearing a multiplicity of m insulated photo-electrically active elements on one face and an electrically con continuous signal plate on the other face. During the operation of the device an optical of the object to be transmitted is proi upon the elements which are at the same time by the cathode my. p cture signals being a in a circuit, external 01' the tube, asociated with the signal plate.

Several diiiiculties have been encountered during the construction of such a tube. In the place since the mosaic screen has to be w it w intothe bulbous portion through the neck portion,ithasbeendiflicult touseof large area. Secondly, it has been found that such mosaic screens, when formed with an fipg plate of mica between the elements and the signal plate, tend to vibrate and also do not lie perfectly flat. Thlrdly, secondary rafiatinn, X-ray and corpuscular, originating (m th w t and the walls of the tube reaches the plate and give rise to spurious signals which distort picture signals. Lastly it has been if diflicult to form a screen having a sensitive photo-electric layer.

During the process of sans! the ts of the mosaic screen with photo-electri active material such as caesium, it may be found difficult to admit just suilicient of this material to give maximum sensitivity without also ting asmall excesswhichfor atimefree in the tube and then settles upon the spaces between elements and impairs the insulation between adjacent elements. A very small excess of the photo-electrically active material can be removed from the interstices between its simply by scanning the mosaic screen with the cathode ray but this removal is temporary only as the material spreads over the layer again when the tube is idle and malts in definition in the picture signals when w i 1 is recommenced.

It is an object of the present invention to provide a cathode ray tube. suitable for purposes of transmlsion, and having a mosaic screen which does not suiier from the above mentioned disadvantages or in which these disadvantages are reduced.

A further object is to provide improved methods of constructing a. cathode ray tube suitable for television transmission.

to the present invention, a cathode ray tube compriw a mosaic screen formed on a pliant sheet, and supporting means which serve tohold the screen substantially flat in the bulbous portion of the envelope.

to the invention in a further aspect, a method of constructing a cathode 'ray tube having a neck portion flaring out into a bulbous portion and having a mosaic screen disposed within said bulbous portion, comprises the steps of forming said mosaic screen upon a pliant sheet, bending or rolling said screen, inserting the bent or screen through said neck portion into said bulbous portion and imbending or unrolling said screen so that it lies substantially flat within said us portion. a

The term fpliant sheet means a sheet capable of being distortedwithout injury to such a degree that the portion adjacent to one edge thereof is to soft X-ray and corpuscular radiation which may arise within the tube.

One of the two plates of impervious material may be used as a support for the whole mosaic screen, and it may extend substantially up to the of the bulbous portion of the tube.

Where the screen structure flts fairly closely tothe walls of the bulbous portion of the tube, the difliculty in photo-sensitizing the mosaic elements, hereinbefore referred to, may be overcome by using, as a photo-electrically active materiaLanalkalimetalsuchas caesium,'havinga slow rate of diflusion in the envelope, by providing in the tube, on the side of the screen opposite to that on which the mosaic elements are mounted; a. material such as silver oxide, having an ailinity, for the photo-electrically active material and admitting the photo-electrically acthrough an angle of not less than 180 deg. relative to the portion adjacent to the oppotive material to the tube on the side of the screen the screen II.

on which the mosaic elements are mounted until these elements have a high photo-electric sensitivity. Although at this stage there may be sumcient photo-electrically active material on the front of the screen to impair the insulation between the mosaic elements, owing to its slow rate of diffusion, only a relatively small quantity will have reached the part of the tube behind the screen, so that the material provided there, which has an amnity for the photo-electrically active material, will still be unsaturated and'will be capable of subsequently absorbing excess photoelectrically active material present in the tube.

Ways of carrying out the invention will be described with reference to the accompanying drawings, in which Fig. 1 shows a cathode ray tube constructed in accordance with the present invention, and Fig. 2 shows a modified form of tube.

Referring to Fig. 1, a cathode ray tube comprises a sealed glass envelope having a bulbous portion I and two cylindrical neck portions 2 and 3. The portion 2 is longer and narrower than the portion 3. During the construction of the tube an electron gun (not'shown) is inserted into the portion 2 before the end of this portion is sealed. The electron gun may comprise, for example, a cathode heater, a cathode, a grid or modulating electrode and a first anode, these electrodes being disposed within the neck portion 2 in the order mentioned, A second anode l is also disposed within the tube partly in the neck portion 2 and partly in the bulbous portion I. This second anode 4 may be, as shown, in the form of a conductive coating on the inner wall of the envelope and a lead 5 is taken to the second anode through the wall of the bulbous portion I. The electron gun is associated with electro-static or electro-magnetic means (not shown) for periodically deflecting the ray produced thereby.

Within the bulbous portion I is also disposed a mosaic screen comprising a layer 6 of, mutually insulated photo-electrically active elements disposed upon one side of a square insulating plate I. The insulating plate I is made of very thin sheet mica of about 4 inches on the side and on the face of the mica plate I opposite to that bearing the elements is an electrically conductive continuous plate 8 of silver. Henceforth this plate will be called the signal plate" of the tube. The photo-electrically active elements do not extend right up to the edge of the insulating plate I.

The insulating plate I, carrying the elements and the signal plate, is attached by metal clips, one of which is shown at Hi, to a circular mica screen II of about 7 inches diameter. The mica screen II is considerably thicker than the insulating plate of mica I. One of the clips I is in contact with the signal plate 8 and is connected to an output lead I2.

Also attached to the mica screen II is a framing plate l3 of mica of the same thickness as framing plate I3 is disposed so that it overlaps the insulating plate I, as shown at I and I6, by about $4; inch all around. The framing plate l3 and the mica screen ll extend nearly up to the inside of the walls of the bulbous portion I of the envelope and are supported by means of helical springs (one of which is shown at 9) The screen II and the framing sealed into the bulbous portion. The side of the framing plate l3 facing the neck portion 2 is coated with an electrically conductive continuous layer II, which should not overlap the signal plate. Thusthe signal plate is substantially enclosed between two sheets of mica, one of these sheets being in two parts I and I3 and the other being the single mica screen II.

The plates I, II and ii are assembled and the silver mosaic layer is formed upon the plate I and the continuous electrically conductive layer I1 is formed upon the plate l3 before insertion into the tube. The assembled plates are then rolled up or bent, inserted into the bulbous portion I, through the neck portion 3, unrolled or unbent, and attached to the three supporting springs 9. Theneck portion is then sealed and the envelope is evacuated. The surface 6 which bears the silver mosaic is then oxidised by admitting oxygen at a low pressure into the envelope and passing a high-frequency discharge through the envelope. Excess oxygen is then pumped out and caesium -is distilled into the envelope to sensitise the oxidised elements photoelectrically. During the distillation process the envelope is baked intermittently for short peiods at about 200 C. and the amount of caesium admitted is chosen to give maximum photo-electric sensitivity, consistent with good insulation between the mosaic elements.

During the operation of the tube an optical image of the object to be transmittedis projected upon the photo-electric layer 6 and this part of the screen is scanned by the cathode ray. It is found that with this arrangement the layer 6 lies in a substantially flat plane and is free from serious vibration. Also, since mica is impervious to soft X-rays and slow-moving electrons, spurious signals, which hitherto appeared in the picture signals at the beginning and ends of lines as aresult of charges acquired by the signal plate from secondary radiation such as soft X-rays and slow-moving electron radiation originating on the wallsof the tube, etc., are substantially eliminated.

Finally it is found that. the photo-electric layer 6 oxidises very uniformly because, owing to the presence of the electrically conductive layer II,,

the photo-electric area has no unscreened edges which concentrate the high-frequency discharge utilised to effect the oxidation. This uniform oxidation of the layer 6 allows a uniformly sensitive photo-electric mosaic of elements to be produced.

The method of avoiding the presence of an excess of photo-electrically active material in the tube will be described with reference tov Fig. 2. In this construction the framing plate I3 is replaced by four narrow strips of mica l8 which cover the edges of the plates 1 all round and which are secured by rivets I 9 to the mica screen II at points lying close to the edges of the plate I. In this way the signal plate 8, the insulating plate I and mosaic layer 6 are held loosely but eifectively flat against the mica screen H which,

fits fairly closely to the walls of the envelope.

The layer 6 and the whole of the back of the screen II are coated, before insertion into the envelope, with a thin layer of silver which is broken up to form a mosaic. Theiayer on the back of the screen is denoted by 20. The whole structure is then assembled within the envelope and the envelope is sealed.

After the usual evacuating and degassing processes, oxygen is admitted to the tube, and the 2,149,455 silver mosaics 6 and 20 are oxidised by producing a high-frequency discharge in the gas. Excess oxygen is then pumped out and caesium is admitted from a side tube 2i, which is sealed on to the bulb I of the tube in such a way that the stream of caesium vapour is directed away from the mosaic towards the walls of the tube, from which it is scattered back over the whole of the face of the screen. In this way it is possible to activate the mosaic layer 6 very uniformly with caesium.

The caesium is admitted in several separate lots, the tube being baked between each until the maximum photo-electric sensitivity is obtained on the mosaic layer 6. At this stage the silver oxide on the front surfaces of the composite plate is saturated with caesium, and there is probably suiiicient excess caesium present to break down the insulation between the individual elements. But owing to the slow rate of diffusion of the caesium vapour through the gap between the screen II and the walls of the envelope, the silver oxide on the elements 20 will still be far from saturated with caesium. Now, by baking the tube gently, or by allowing it to lie idle for long enough 'at ordinary temperatures,

the excess caesium from layer 6 will be collected by the silver' oxide mosaic 20, leaving the layer 6 with good insulation between the elements, and at the same time highly sensitive photo-electrically.

We claim:

1. A cathode ray tube comprising an envelope having a bulbous portion, a mosaic screen formed on a plurality of separated pliant sheets of insulating material superimposed one on another, and a plurality of resilient supporting means which serve to hold said screen substantially flat within said bulbous portion.

2. A cathode ray tube comprising an envelope.

having a bulbous portion, a mosaic screen comprising a pliant sheet of insulating, material bearing on one side a large number of mosaic elements, a second pliant sheet of insulating material disposed on the other side of said first sheet. a pliant metal signal plate disposed between said sheets and means which serve to maintain said sheets pressed together, and supporting means which serve to hold said screen subtantially flat within said bulbous portion.

3. A cathode ray tube comprising an envelope having a bulbous portion, a mosaic screen formed on a pliant sheet, and a plurality of springs distributed round the edge of said screen, connected between said screen and said bulbous portion, and serving to support said screen substantially flat within said bulbous portion.

4. A cathode ray tube comprising an envelope having a bulbous portion, apliant sheetof insulating material, a plurality of springs distributed around the edge of said sheet, connected between said sheet and said bulbous portion, and serving to support said sheet substantially flat within said bulbous portion, a second pliant sheet of insulating material disposed adjacent to said first-mentioned sheet, a pliant metal signal plate disposed between and. covered by said sheets, means serving to maintain said sheets pressed together, and a plurality of mosaic elements mounted on the exposed side of said secondmentioned sheet.

5. A cathode ray tube comprising a mosaic screen having a photo-electrically-active coating and a planar electrically-conductive signal plate, and two planar sheets of mica substantially impervious to soft X-ray and corpuscular'radiation and disposed one on each side of, so as substantially to enclose, said signal plate.

6. A cathode ray tube comprising an envelope, a mosaic screen having a planar electricallyconductive signal plate, two planar sheets of an insulating material substantially impervious to soft X-ray and corpuscular radiation, said sheets being disposed one on each side of, so as substantially to enclose, said signal plate, and a plurality of photo-electric mosaic elements supported, and insulated from one another and from said signal plate by one of said sheets, and means connecting the other of said sheets to said envelope and serving to support said screen.

7. A cathode ray tube comprising an envelope, a mosaic screen having a photo-electrically-active coating and an electrically-conductive signal plate and two sheets of an insulating material substantially impervious to'soft X-ray and corpuscular radiation and disposed one on each side of, so as substantially to enclose, said signal plate, at least one of said sheets extending substantially up to the walls of said envelope.

8. A cathode ray tube comprising a mosaic screen having an electrically-conductive signal plate, two sheets of insulating material disposed one on each side of said signal plate, one of said sheets being larger than the other of said sheets and so disposed that the border of the larger of said sheets extends beyond the edge of the smaller of said sheets, a framing element secured to said larger sheet and overlapping the border of said smaller sheet, and a plurality of electricallyconductive mosaic elements mounted on the .exposed side of at least one of said sheets.

9. A cathode ray tube comprising an envelope, a mosaic screen having an electrically-conductive signal plate, two sheets of insulating material disposed one on each side of said signal plate, one of said sheets being larger than the other of said sheets and so disposed that the border of the larger of said sheets extends beyond the edge of the smaller of said sheets, a framing element secured to said larger sheet and overlapping the border of said smaller sheet, and a plurality of electrically-conductive mosaic elements mounted on the exposed side of at least one of said sheets, and means connected between said larger sheet and said envelope and serving to support said screen.

JAMES DWYER McGEE.

GEORGE STANLEY PERCIVAL FREEMAN. 

